The Internet's traffic growth shows no sign of abating. To keep pace with the growth, the capacity of a single-rack rooter has grown about threefold every 18 months. But the energy-per-bit efficiency has only decreased at a rate of about 10% per year since 2000. Since the power dissipation within a rack cannot substantially increase due to the present limitations of air cooling, the energy-per-bit efficiency will quickly become the ultimate capacity limiting factor for future routers and data center networks.
People have turned to optics for solutions. Photons as an information carrier are much more power efficient than electrons. Parallel VCSEL (vertical-cavity surface-emitting laser) links have already been deployed for board-to-board and rack-to-rack communications. The problem mainly lies in switching. Take the interconnect system, of a data center network for example. It usually contains two or three tiers and electronic switching is performed in each tier. This interconnect architecture introduces as many as six O/E and E/O conversions and these conversions consume a large amount of power. As a result, they greatly diminish photonics' potential to improve the energy-per-bit efficiency in data center networks.
If the interconnect of a data center network can be implemented with optics, the E/O and O/E conversions inside the switch will disappear and tremendous power saving can be achieved. AWGRs (Arrayed Wavelength Grating Router) offer a promising solution. An AWGR is a passive device and consumes almost no power. Using diffraction grating, an AWGR routes optical signals based on their wavelengths and N2 flows can traverse the device simultaneously (N is the port count and also the number of wavelengths). In contrast, only N flows can pass through an N×N electronic crossbar at any given time.
Although AWGRs have become the center piece of many optical interconnect proposals, this technology has one fundamental limitation: poor scalability. To increase the port count of an AWGR, we have to increase the number of wavelength channels by reducing channel spacing. Unfortunately, this will lead to higher channel crosstalk. A realistic port count for an AWGR is likely less than fifty. A large data center network, however, may need to interconnect a thousand racks, or more. Solving the scalability issue has become a prerequisite for using the AWGR technology for large routers or data center networks.